What I have learnt about geo thermal heat pumps

I am in the process of investigating the replacement of my oil furnace by a
geo thermal heat pump. This is intended to be a summary of the things I
have learned during this process.
For background, I currently have a 4 ton A/C unit and a 92000 BTU oil
furnace, and live in a 3000 square foot house (built in 1978) near the shore
in New Jersey.
1) Buried ground loop HP.
The loop is made out of plastic tube, and is to be buried in about a 5 foot
trench. Joins in the tubing HAVE to be done by gluing, and you need about
600 feet of tubing for each ton. So in my case I need 3000 feet of tubing,
which translates as 3000 feet of trench. Now the tube can be laid on one
side of the trench, looped at the far end, and returned on the other side of
the trench. Instantly dropping the trench length to 1500 feet. Then you
can fill the trench with about one foot of fill, and repeat the above
process. This cuts the trench length to 750 feet, which is still a lot of
trench.
The problems are that the tube is filled with alcohol to reduce the freezing
temperature, and they have to be leak tested before the trench is finally
filled. This also appears to have the lowest efficiency of any of the
systems.
2) Ground water Loop HP.
In this system two wells are drilled, and water is pumped out of one well,
through the HP, and back into the second well. This has higher efficiency,
but well drilling suffers from serious cost problems in NJ. Also the
injection well may silt up, giving serious maintenance problems (at the very
least). When it works, its very good. This system skips one heat exchange
from system #1.
3) Direct exchange HP.
In this system, the actual refrigerant is passed through a ground loop.
This skips one complete heat exchange from #1, thereby increasing the
efficiency quite a bit. The system I am looking at needs a 40 by 50 foot
pit, 8 feet deep, with refrigerant run through copper tube. In this case
the major problem is the size of the pit, and the potential for the copper
to corrode; so you would then have to re-lay the pit and replace the
refrigerant. Under reasonable conditions the copper will last 50 years.
Also if a leak develops in the copper tube, you will have a serious problem
locating it.
You will also get a choice of refrigerant. I would prefer to have R22 (I
think that's right) for the next 10 years, and about that time switch to a
drop in replacement. R22 is to be phased out in 2020, and I don't have much
faith that the other alternatives have been fully tested yet.
In my case it is estimated that I will save about $2000 per year over my
current expenses (Oil 2500 per year, hot water $600 per year and A/C $300
per year). I am seeing cost estimates ranging from $10000 to $30000 for
everything. My guess is that I can get a reasonable system for $15000, with
a payback of 5 years at current prices. If the price of Nat Gas and Oil
doubles, my payback will be about 3 years.
I should mention that I will also get a credit of $500 per ton from the
state for an acceptable system.
Anything you can add would be appreciated.

IIRC, here they generally go 700 ft /ton capacity--thats a 350 trench there
and back.
1in pvc at 5 ft depth, and the pipe is filled 50% methanol--in a 3 ft wide
trench.....
No putting one loop on top the other, this probly is a no no--as individual
trenches are spaced ~15 ft apart--you need area coupled with the
earth..........
So for a 4 ton unit, you go on and figure on a total area 60 ft wide by 350
ft long.
Disclaimer :
This is only my point of view from talking to a handful of local installers
here in the pacific northwest and reading here and there on the net--they is
all just a "rule of thumb type" of view............
========
Since the local terrain where I reside ( solid rock and mountanous
topography ) doesnt lend itself well to buried loops, I have basically
hacked in a 3.5 ton system where the geothermal is secondary to the main 5
ton air-source unit........a 45,000 gal swimming pool is used for cooling in
summer, heating the pool by rejecting waste heat into it, this also
providing a supplemental heat source in the winter when the outside air
temps dip below the 40 deg or so where efficency of air source unit drops
off steeply--supplemental water can be added at that setpoint by going to
"pump and dump" from the domestic 3 hp well, fresh water simply overflowing
the swimming pool.
Now, well to note there is little advantage in earth coupled systems until
the outside air temp drops below that where air source heat pumps operate
efficently--in a traditional air-source heatpump system, this is the point
where electric or fuel based auxilaary kicks in.
I have but few automatic controls as of yet, other than on the main unit,
and the residence is ~ 7300sf....if I was to have a need to sell in a hurry,
likely the geo system would need removed / abandoned due to probable code
issues and to the fact the average joe needs to have more than just a few
clues in order to actively controll it in order make it work effectively.
This would probably be called some kind of a hybrid system, as far as I
know, none are on the market as of yet, and I dont foresee them to be
readily available in the foreseeable future.
So, my recomendation is to go with all ground coupling, so long as you have
enough real estate that lends itself to installation of the ground loops.
Cheers,

One of the installers I talked to said that he installed a water drip feed
pipe 'close' to the buried coils, so that a little water could be 'added' to
increase coupling if needed. This may be a good idea, but it would really
add to the confusion of the next owner of the system. Has anybody else ever
come across this idea?

I think somebody said 1/4 inch copper was all that is needed. But I would
like to have that confirmed before I started any serious work.

The real problem is that soil quality can lead to corosion of the copper
pipe in a period of about 5 years. I was told that the manufacturer wanted
a soil sample before sending the buried copper components, and also that in
some cases special 'fill' is needed around the copper.

a
shore
foot
tubing,
of
freezing
efficiency,
very
exchange
problem
much
with
Talked to another vendor this morning. He estimated $5000 per ton
completely installed. His version is to drill two 800 feet bores (for a 5
ton unit) and fill then with plastic pipe in grout. Sounded good while I
talked to him. He claimed to have a 6 month backlog, and that Heat Pumps
was all he did.
It appears that there are lots of small shops assembling heat pumps from off
the shelf components (after all they are just A/C units with a few
imporvements) And they all have different stories and backgrounds. It
would be very easy to get screwed, and I'm not sure I have enough
information to make an intelligent choice.

In my case I have an average temp of 31 degrees for the three months of
winter, so an Air source heat pump is just not good enough.
I estimate that I would save $2000 per year using a heat pump at current
energy prices. And the price of oil and gas is rising much faster than the
price of electricity, so that in 5 years I could be saving $3000 per year.
Take my estimated economics:-
Cost of heat pump 15000
Less cost of gas furnace 4000
Less NJ rebate 2500
giving a real cost difference of $8500. So my payback time is just over 4
years.

George,
Thanks for the postings. I'm going through the same process right now.
I live not far from you, just across the Delaware in New Hope.
I've been taking quotes for replacing my oil burner and A/C unit.
Combined price is roughly $12k so payback would be real quick if the
ground source heat pump was truly as efficient as they say.
My property is all rocky and up on a hill, so horizontal is out and
conventional vertical would be inefficient because of the well depth
necessary. So I'm looking at a direct exchange system (i.e. the copper
pipes in vertical boreholes) but will have to get the earth checked for
acidity to avoid corrosion problems you mentioned.
Have you found a good contractor in the area that does DX systems? The
closest one I've found is out near Harrisburg.

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